A plastic optical fiber is composed of a core as a light passage and a clad to cover the core. The clad has a smaller refractive index than the core. In general, there are a glass optical fiber and a plastic optical fiber (POF). The core of the glass optical fiber is composed of quartz or glass. Examples of the glass optical fiber are an entire quartz optical fiber in which the core and the clad are made of quartz, a polymer clad optical fiber in which the core is made of quartz and the clad is made of silicone or polymer fluoride, and a multi-component optical fiber having the glass core and the glass clad. On the other hand, the core and the clad of the POF are composed of plastic material.
For the purpose of decreasing the transmission loss of the quartz optical fiber and the POF, various kinds of structures of the optical fiber are proposed. For example, the clad has two layers (core and clad) in which the refractive index in the core gradually changes as the distance from the center of the core, and the refractive indices in the core and the clad, and the diameter of the clad are adjusted to decrease the transmission loss. In addition, “Usage of Plastic Fiber and Important Notices”, (issued from Optronics Co., Ltd., 1992), in page 87, describes an optical fiber in which the refractive indices in the clad and the core, and the diameter of the core satisfy a certain condition.
Compared with the quartz, the plastic material as the optical transmission medium has advantages in formability, weight, low manufacture cost, bending property, resistance to shock, and so forth. Because of the larger transmission loss than the glass optical fiber, the plastic optical fiber is not suitable in transmitting optical signals for a long distance. The plastic optical fiber, however, enables a large diameter of the core part (several micrometers or larger, for example). Thus, with the plastic optical fiber, it is not necessary to connect the plastic optical fiber to an optical device for dividing or connecting the optical fibers with high precision. Therefore, the plastic optical fiber has merits, such as facility in connection, facility in fiber terminal process, non-necessity for core alignment with high precision. Moreover, the plastic optical fiber has various merits such as low danger to prick into human body, easy construction, high resistance to vibration and low price. Accordingly, it is planned to utilize the plastic optical fiber not only as household and automobile purposes but as a short-distance, high-capacity cable such as inner wirings for high-speed data processing device and a digital video interface (DVI) link.
The POF for public purpose is fixed to an attachable connector, so the plastic optical fiber for such purpose has a large diameter for the purpose of increasing the permissive range to position deviation in connection. The plastic optical fiber with a large diameter can increase the permissive range in connection with a light source (light emission element, light emission device) with a large numerical aperture. But the angle of emergence of the signal light from the fiber edge increases as well. Reducing the gap from the optical fiber edge to the light receiving element (light receiving device) for the purpose of reducing the angle of emergence requires high alignment precision, so reducing the gap will lose the advantage of the POF in terms of facility in connection. In order to deal with such problem, Japanese Patent Laid-Open Publication (JP-A) Nos. 2002-311260 and 2002-350654, and the above mentioned reference (“Usage of Plastic Fiber and Important Notices”) discloses the solutions to locate a light-focusing element between the optical fiber edge and the light receiving element, and to increase the size of the light receiving element.
The light-focusing element disclosed in the above references, however, causes Fresnel loss at the interface of the optical elements and increase in manufacture cost due to increase in optical parts. Moreover, increasing the size of the light receiving element increases the manufacture cost and decreases the sensitivity due to increase in the dark current. Thus, it is desired to realize optical transmission at a low cost without losing the advantages of the optical fiber with a large diameter.
An object of the present invention is to provide an optical connection method and an optical transmission apparatus to realize excellent connection between a plastic optical fiber having a large diameter and a light receiving element, and thus possible to improve the transmittance from the light emission element and the light receiving element through the plastic optical fiber.